In the field of robotics, and in particular in the field of wearable robotics, the use of elastic actuators is frequent (see, for example, Veneman et al., “A Series Elastic- and Bowden-Cable-Based Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots”, The International Journal of Robotics Research 2006 25: 261; Pratt et al., “The RoboKnee: An Exoskeleton for Enhancing Strength and Endurance During Walking”, Proceedings of the 2004 IEEE International Conference on Robotics & Automation New Orleans, April 2004; Torres-Jara et. al, “A simple and scalable force actuator”, Proceedings of 35th International Symposium on Robotics, Paris, France, 2004; and Paine et al., “A New Prismatic Series Elastic Actuator with Compact Size and High Performance”), wherein an elastic element is arranged between the actuator and the actuated mechanical element (see, for example, Pratt et al., “Series elastic actuators” in Proc. IEEE Intl. Conf. Intell. Robots Syst., Pittsburgh, Pa., 1995, pp. 339-406). For example, as described in U.S. Pat. No. 5,910,720, there are several reasons behind the use of this type of actuation especially in robotics. It definitely implies a series of advantages that can be summarised in the following points:                low exit impedance on the entire frequency spectrum;        possibility of controlling the exit impedance through software;        reducing the energy consumption;        high force/mass ratio;        high power/mass ratio;        inherent compliance in case of impact.        
These types of actuators may be linear or rotary. Both types of actuators may be implemented with elastic linear or torsional elements (generally springs or assembled devices comprising springs).
One of the criticalities when it comes to implementing this type of actuators lies in the choice and construction or the elastic element.
The main specifications, which vary as a function of the application the actuators are used for, characterizing an elastic element to be used in an elastic actuators are:                Rigidity;        Maximum admissible load;        Admissible rotation or displacement;        Weight;        Overall dimension (shape).        
With reference to the case of rotary actuators, whose field is a more specific object of the present invention, and thus elastic elements in which the transmission of a torsional stress is carried out, the prior art provides for various embodiments.
Generally speaking, a torsional elastic element may be obtained by using one of the following elements:                Wire helical torsional spring;        Machined (from an integral block) helical torsional spring;        Spiral torsional spring;        Mechanism which converts the linear spring action in a torsional response;        Torsional response custom element; The use of wire helical torsional springs implies the following problems:        Low rigidity with respect to the requirements set by robotic applications;        Difficulty of interfacing with the elements in series therewith. The torque is transmitted through contact between an element and the wire of the spring which, in case of high deformations, slides on the surface of contact with the element;        Difficulty to obtain the bi-directionality of the response. In order to obtain an element capable of worling in both directions of rotation it is necessary to create a mechanism provided with at least two springs with ensuing increase of complexity, mass and dimensions;        Contact between coils during motion;        Remarkable overall bulk caused by the presence of spring lever arms.        
The machined springs used as torsional springs overcome some of the aforementioned drawbacks (see, for example, Helical Products Company, Inc. http://www.heli-cal.com). Specifically, they are metal cylinders in which there is formed a helical recess with one or more principles, so that the cylinder takes on a helical shape.
One of the main advantages of this type of springs lies in the possibility of providing the ends thereof (to become interface areas with the elements to which they are fixed) with different shapes and with fixing systems which allow forming various couplings (threaded holes, threaded ends, notched profiles etc.).
However, alike the wire springs these springs have a preferential direction of rotation and this makes them not suitable for use in applications in which there is expected the application of torques in both directions of rotation and it is required an identical torsional response in both directions.
Also the use of spiral springs allows overcoming some drawbacks of the wire helical springs but there remains the impossibility to obtain a two-directional response without using more than one spring and a connection mechanism.
By using linear springs in an assembled device, which converts the linear response thereof into a torsional output response, there can be obtained a two-directional response with desired rigidity and transmissible torque characteristics. The drawbacks related to this type of solution mainly lie in the large overall dimension required for the implementation of the entire assembly.
Among known examples of torsional springs formed starting from a suitably machined metal element, with the aim of conferring the desired properties to the element, the one disclosed in Lagoda et al., “Design of an electric Series Elastic Actuated Joint for robotic gait rehabilitation training” Proceedings of the 2010 3rd IEEE RAS EMBS International Conference on Biomedical Robotics and Biomechatronics, The University of Tokyo, Tokyo, Japan, Sep. 26-29, 2010, is an elastic actuator used in walking rehabilitation. The elastic element used in the actuator is obtained from a plate-like steel body in which there are formed two spiral recesses. The element reveals some problems in connection with hysteresis, the contact between the coils that limits the applicable load and the relatively high difference between the rigidity simulated with FEM analysis and actual rigidity.
An embodiment analogous to the one described above is in Wang, Shiqian, et al. “Efficient Lightweight Series Elastic Actuation for an Exoskeleton Joint”. The shape of the elastic element is generally similar to the previous one though with increased torsional rigidity and the attempt to overcome the problems of hysteresis and contact between turns. Stienen et al., “Design of a rotational hydro-elastic actuator for a powered exoskeleton for upper-limb rehabilitation,” IEEE Trans. Biomed. Eng., vol. 57, no. 3, pp. 728-735, March 2010, discloses a spring similar to the previous ones used in a hydro-elastic actuator for the rehabilitation of an upper limb.
Another type of torsional spring obtained by machining a steel element is described in Sergi et. al, “Design and Characterization of a Compact Rotary Series Elastic Actuator for Knee Assistance During Overground Walking”, in Proc. IEEE Int. Conf. on Biomed. Rob. and Biomech., pp. 1931-1936, 2012; in this case, a metal disc is excavated so as to obtain spokes in the shape of laminar coils which join a hub and an external rim. Being disc-shaped, this element has a high diameter/height ratio. The maximum torque applicable is limited by the occurrence of contact between the coils.
Patent publication WO2008US61560 discloses a torsional element in which the elastic response is obtained by joining two parallel flanges with S-shaped elements. The elements for joining the two flanges are bars folded and fixed to the elements for input and output of the torque in the system. According to a simplified variant, shown in US20070698811, the connection between the flanges for the input and output of the torque is obtained by using straight bars and not S-shaped ones. These are complex systems which require the assembly of a plurality of parts hence implying various complications. The connections between the elements must be stable and free of play, so as to avoid a torque transfer mode (angle/torque characteristic) not repeatable or different from the desired one, and the occurrence of unexpected stress potentially causing damage to the structure. In addition, the machining of all elements should be extremely accurate so as to avoid the occurrence of residue stresses after assembly which may modify the characteristic of the elastic element or reduce the resistance thereof.
Again, the aforementioned patent U.S. Pat. No. 5,910,720 shows a torsional spring obtained with an element having cross-shaped sections, and thus the use of plates as the basic torque transfer element. However, the cross represents a configuration of plates working substantially “in parallel”, hence requiring, with the aim of obtaining a high transmissible torque/rigidity ratio, i.e. high transmissible torque but limited rigidity (high deformability), the use of very thin plates (excessive stresses) or the increase in the longitudinal dimension of the object (excessive overall dimension).
Another known torsional response element is disclosed in patent publication EP1244817. In this case the torsional property is obtained by forming, on a cylindrical ring in charge of the transmission of the torque between an input element and an ouput element, a series of recesses that with a radial development define a plurality of segments, in turn developing according to radial planes, that is passing thorough the torsion axis. The whole device is realized in multiple parts that require a rigid and precise mutual connection. Moreover, obtaining the recesses in the radial directions requires to carry out a number of cuts on the ring body, with a resulting constructive complication deriving from the necessity of changing over and over the mutual placement between the body and the cutting tool. Furthermore, being the transmissive segments arranged in a ring-like body, they remain displaced from the torsion axis, and thus they tend to become deformed in flexion, realizing an unsatisfactory ratio between transmissible torque and rigidity.